The first Australia-wide monograph of Amphiroa (Corallinaceae, Rhodophyta) based on the morphological–anatomical species concept has confirmed the occurrence of nine species (A. anceps, A. beauvoisii, A. crassa, A. exilis, A. foliacea, A. fragilissima, A. gracilis, A. klochkovana, A. tribulus) and provides a basis for future molecular-systematics studies. A comparative analysis of 285 specimens from 121 localities, along with type material examinations, yielded clear evidence that nine characters associated with the vegetative system and tetrasporangial conceptacles were diagnostically significant at species level. Detailed accounts of each species are presented, along with an identification key, data on type specimens, species comparisons and biogeographic notes. A. foliacea is epitypified. Brief accounts of 34 additional taxa reported from Australia and at some stage ascribed to Amphiroa are included. Galaxaura versicolor and Amphiroa galaxauroides are heterotypic synonyms of A. anceps; recognition of A. anastomosans, A. ephedraea and A. nobilis as distinct species requires further evaluation; and A. dilatata and A. gaillonii are species of uncertain status. Amphiroa ephedraea is lectotypified. The remaining 27 names involve nomina nuda, illegitimate names, orthographic variants or taxa now excluded from Amphiroa.